Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, also heat-sensitive printing plate precursors have become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross linking of a polymer, heat-induced solubilization, or particle coagulation of a thermoplastic polymer latex.
The most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically comprises an oleophilic binder, e.g. a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working), by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Typical examples of such plates are described in e.g. EP-As 625 728, 823 327, 825 927, 864 420, 894 622 and 901 902. Negative working embodiments of such thermal materials often require a pre-heat step between exposure and development as described in e.g. EP-A 625 728.
Negative working plate precursors which do not require a pre-heat step may contain an image-recording layer that works by heat-induced particle coalescence of a thermoplastic polymer latex, as described in e.g. EP-As 770 494, 770 495, 770 496 and 770 497. These patents disclose a method for making a lithographic printing plate comprising the steps of (1) image-wise exposing an imaging element comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat and (2) developing the image-wise exposed element by applying fountain and/or ink.
EP-A 1 342 568 describes a method of making a lithographic printing plate comprising the steps of (1) image-wise exposing an imaging element comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat and (2) developing the image-wise exposed element by applying a gum solution, thereby removing non-exposed areas of the coating from the support.
WO2006/037716 describes a method for preparing a lithographic printing plate which comprises the steps of (1) image-wise exposing an imaging element comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat and (2) developing the image-wise exposed element by applying a gum solution, thereby removing non-exposed areas of the coating from the support wherein said thermoplastic polymer particles have an average particle size between 40 nm and 63 nm and wherein the amount of the hydrophobic thermoplastic polymer particles is more than 70% and less than 85% by weight, relative to the image recording layer.
EP-A 1 614 538 describes a negative working lithographic printing plate precursor which comprises a support having a hydrophilic surface or which is provided with a hydrophilic layer and a coating provided thereon, the coating comprising an image-recording layer which comprises hydrophobic thermoplastic polymer particles and a hydrophilic binder, wherein said hydrophobic thermoplastic polymer particles have an average particle size in the range from 45 nm to 63 nm and wherein the amount of the hydrophobic thermoplastic polymer particles in the image-recording layer is at least 70% by weight relative to the image-recording layer.
EP-A 1 614 539 and EP-A 1 614 540 describe a method of making a lithographic printing plate comprising the steps of (1) image-wise exposing an imaging element as disclosed in EP-A 1 614 538 and (2) developing the image-wise exposed element by applying an aqueous, alkaline solution.
A first problem associated with negative-working printing plates that work according to the mechanism of heat-induced latex-coalescence is the complete removal of the non-exposed areas during the development step (i.e. clean-out). An insufficient clean-out may result in toning on the press, i.e. an undesirable increased tendency of ink-acceptance in the non-image areas. This clean-out problem tends to become worse when the particle diameter of the thermoplastic particles used in the printing plate decreases, as mentioned in EP-As 1 614 538, 1 614 539, 1 614 540 and WO2006/037716.
A decrease of the particle diameter of the hydrophobic thermoplastic particles in the imaging layer may however further increase the sensitivity of the printing plate precursor. The rather low sensitivity of negative-working printing plates that work according to the mechanism of heat-induced latex-coalescence is a second problem to be solved. A printing plate precursor characterized by a low sensitivity needs a longer exposure time and therefore results in a lower throughput (i.e. lower number of printing plate precursors that can be exposed in a given time interval).
According to the unpublished EP-A 06 114 473 (filed 2006-05-24) a good clean-out is obtained, even with printing plate precursors comprising thermoplastic particles having a particle diameter of less than 40 nm, when the amount of infrared light absorbing dye, without taking into account an optional counterion, is more than 0.80 mg per m2 of the total surface of the hydrophobic particles.
According to the unpublished EP-A 06 114 475 (filed 2006-05-24) a good clean-out is obtained when said amount of the infrared light absorbing dye is more than 0.70 mg per m2 of the total surface of the hydrophobic particles, when the precursor is developed in an alkaline developer. A possible disadvantage of this invention may be a too high absorption of infrared light by the coating, due to the high amount of infrared light absorbing dye present in the image-recording layer, resulting in a low sensitivity.
To enable a visual inspection of the exposed image on the printing plate, after exposure and processing of the precursor, colorants are often added to the coating of the precursor. After removal of the non-exposed areas of said coating by the processing, the colorants in the exposed areas provide a visual image. Said colorants can be pigments or dyes. The lithographic printing plate precursor used in the method of producing a printing plate described in WO2006/037716 comprises preferably a pigment, more preferably a pigment having a hydrophilic surface. EP 1 524 112 describes a lithographic printing plate precursor comprising a contrast layer wherein said contrast layer comprises a colorant capable of providing a visible image after exposure and development of the precursor and wherein the image recording layer is substantially free of the colorant. According to the unpublished EP-A 05 109 781 (filed 2005-10-20) the lithographic printing plate precursor may comprise amino-substituted tri- or diarylmethane dyes as contrast dyes. WO2006/005688 discloses dyes which, combined with specific additives, only slightly color the coating but become intensely colored after exposure. The unpublished EP-A 05 105 440 (filed 2005-06-21) and PCT/EP2006/063327 (filed 2006-06-20) disclose infrared light absorbing dyes providing a print-out image after exposure to infrared light.